1) Field of the Invention
The present invention relates generally to an apparatus for sensing the velocity of metallic parts such as gears and, in particular, to a modular sensor assembly for such an apparatus.
2) Description of the Prior Art
In order to control the acceleration or deceleration of a wheeled vehicle, such as a car, truck or diesel locomotive, it is necessary that the speed of the wheel be measured either directly or indirectly. In either accelerating or decelerating control systems, it is desirable to apply power or brakes to the driving wheels in such a manner that the wheels are on the verge of slipping or breaking loose at all times. Indirect velocity measuring systems have not proven satisfactory because of varying road surface conditions and changes in the coefficient of friction between the wheel surfaces and the road.
A number of prior art devices and techniques have been used to measure the speed of the drive wheels either directly by measuring the speed of the drive gear. These include Hall effect, elecromagnetic and optical devices. Because of the cost of the Hall effect devices and the need for a clean environment for the optical devices, electric electromagnetic sensors have usually been the choice. One such sensor assembly suitable for use with such a control system is disclosed in U.S. Pat. No. 3,721,968 issued to Gee, the entire specification of which is hereby incorporated by reference.
However, a number of practical disadvantages have plagued such electromagnetic sensors. First, the electromagnetic sensor is sensitive to spacing variations between the sensor and the moving part. Second, the signal strength is dependent on the speed of the moving part and very low speeds may render the sensor signal useless. However, with the advent of integrated digital circuits, it has now become possible to measure near zero speeds. However, the problem of sensitivity to spacing variations remains.
There are three major sources of spacing variations between the sensor assembly and the moving part. First, the moving part or gear itself is often not concentric. For example, variations of 20-50 mils would be typical. Second, there will be variation in the dimensions of the sensor itself also due to practical manufacturing restraints. Third, installation of the sensor assembly itself by a technician may also introduce dimensional errors. Thus, it has been the common practice to individually assemble and calibrate each sensor during manufacture. Once in the field, the technician then may find it necessary to insert shim spacers between the housing and the traction gear box to compensate for idiosyncrasies of that particular unit. As a result, installation of a new or replacement speed sensor assembly often requires up to 8 hours to install.
It has thus become desirable to develop a speed sensor assembly for a velocity control system that will be suitable for use with such a system while, at the same time, eliminating the prior art problems of recalibration and the expense each time the sensor must be replaced.